Turbine Nozzle Cascade Research Articles

Application of the Parametric Method for Profiling the Interblade Channels in the Nozzle Cascades of Axial-Flow Turbine Machines

Examples of successful application of the technique for parametrically profiling the blades of axial-flow turbine machines are given. The interconnection between the cascade parameters and the profile parameters that are used for representing the blade profile’s shape is described. The influence of each of the considered parameters on the flow characteristics downstream of the cascade, in particular, on the kinetic energy losses and working fluid outlet angle, is analyzed. The initial cascade is optimized for enhancing its aerodynamic efficiency by improving the streamlining of the blade suction side and trailing edge. The optimization results were confirmed by calculation and experimental investigations. By using the developed method, the problem of designing the turbine nozzle cascade for its operation under wet steam conditions with high-efficient removal of moisture from the steam turbine flow path has been solved. Based on the elaborated recommendations for controlling droplet flows through modifying the blade profile’s shape, the profile and cascade have been designed that make it possible to obtain highly intensive growth of liquid film thickness and flowrate on the interblade channel surfaces in the places from which it can be removed through the interchannel separation slots. The accomplished set of calculation and experimental investigations has confirmed the effectiveness of the selected approach.

Investigation on effects of shock wave on vortical wake flow in a turbine nozzle cascade

Vortical wake flow and its interaction with shock waves in a transonic turbine nozzle cascade were investigated by experimental and numerical methods. Measurements including aerodynamic performance and Schlieren photography were conducted on a validating turbine nozzle cascade. Numerical simulations were performed on both the validating turbine cascade and a single-passage turbine nozzle using two numerical schemes, the RANS and the detached eddy simulation (DES) simulations. The comparison of the wake flow patterns indicated that the DES calculation was more adaptable in simulating the vortical wake flow than the RANS method. Based on the DES results, it was found that, after crossing the shock wave, the vortical wake flow had three changes in its flow characteristic: 1) the vorticity increase in the vortex core; 2) the reduction of the distance between two adjacent vortices with the same direction; 3) the deflection of the entire vortex structure including the vortex core and its preparation stage. Those changes were explained by aerodynamic principles. To further validate the findings, the investigation was carried out at a subsonic condition, and the disappearance of those changes was extra evidence to support that the interaction of the shock wave contributes to the vortex changes.

Thermodynamic losses of hetero and homogeneous condensation in steam turbine cascades

The approach used in this paper for the condensation of flowing steam is based on the homogeneous condensation of steam and also on the heterogeneous (binary) nucleation of water droplets with a representative chemical impurity NaCl. The physical and mathematical models are briefly described. The homogeneous and binary nucleation numerical models are then used for the calculation of steam flow with condensation in the two-dimensional nozzle-blade cascade of the first wet stage of the low-pressure part of a fossil-fuelled condensing 200-MW steam turbine and in a similar cascade of the low-pressure part of a nuclear 1000-MW turbine. Results of the calculations of the flow in the cascades and of the thermodynamic loss are presented and analysed. It is recommended to situate the salt solution zone of NaCl upstream of a turbine nozzle cascade, to use a lower expansion rate on the cascade, and to realise there, primarily, a heterogeneous process (binary nucleation of steam and NaCl) for the initial condensation.

Performance of 2D scheme and different models in predicting flow turbulence and heat transfer through a supersonic turbine nozzle cascade

A number of turbulence models were employed to investigate the heat transfer and aerodynamic characteristics through a nozzle cascade of a high-pressure gas turbine. Isentropic Mach number and Nusselt number around the vane were predicted and compared to existing experimental data obtained at a supersonic flow condition. According to the result presented by different models, possible source of the prediction error was identified; and the performance of different turbulence closures in predicting the heat transfer characteristics around the vane was discussed. It shows that the calculated heat transfer result was affected directly by the predicted turbulence transportation throughout the boundary layer. Finally considering the computational cost and the performance of the models, the suitable model(s) are recommended for the further 3D applications.

Pneumatic measurements downstream of a radial turbine nozzle cascade

This paper deals mainly with pneumatic measurements on a radial turbine nozzle cascade. The full radial cascade guarantees the exit flow field periodicity downstream of it. A special traversing mechanism with a five — hole conical probe moving along a circular path behind the cascade was used for flow field investigation in this type of cascade with very low aspect ratio. The analyses of results of 2D and 3D pneumatic measurements including loss coefficient values are presented.

Design and Testing of a Transonic Linear Cascade Tunnel With Optimized Slotted Walls

In linear cascade wind tunnel tests, a high level of pitchwise periodicity is desirable to reproduce the azimuthal periodicity in the stage of an axial compressor or turbine. Transonic tests in a cascade wind tunnel with open jet boundaries have been shown to suffer from spurious waves, reflected at the jet boundary, that compromise the flow periodicity in pitch. This problem can be tackled by placing at this boundary a slotted tailboard with a specific wall void ratio s and pitch angle α. The optimal value of the s-α pair depends on the test section geometry and on the tunnel running conditions. An inviscid two-dimensional numerical method has been developed to predict transonic linear cascade flows, with and without a tailboard, and quantify the nonperiodicity in the discharge. This method includes a new computational boundary condition to model the effects of the tailboard slots on the cascade interior flow. This method has been applied to a six-blade turbine nozzle cascade, transonically tested at the University of Leicester. The numerical results identified a specific slotted tailboard geometry, able to minimize the spurious reflected waves and regain some pitchwise flow periodicity. The wind tunnel open jet test section was redesigned accordingly. Pressure measurements at the cascade outlet and synchronous spark schlieren visualization of the test section, with and without the optimized slotted tailboard, have confirmed the gain in pitchwise periodicity predicted by the numerical model.

On Vortex Formation in the Wake Flows of Transonic Turbine Blades and Oscillating Airfoils

This paper demonstrates similarities between the vortex shedding from blunt trailing-edge transonic turbine nozzle blades and from oscillating airfoils and bluff bodies. Under subsonic conditions the turbine nozzle cascade shed wake vortices in a conventional von Kármán vortex street. This was linked with a depressed base pressure and associated energy separation in the wake. Under transonic conditions a variety of different shedding configurations was observed with vortices shedding and pairing in several different ways. Similarities are addressed between the observed structures and those from vortex shedding in some other physical situations, such as the vortex wakes shed from cylinders and airfoils in sinusoidal heaving motion in low-speed flow. The established field of vortex-induced vibration has provided a developed classification scheme for the phenomena observed. The paper has brought together three previously independent fields of investigation and, by showing that the three are essentially related, has provided the basis for a new synthesis.

Measurement and Computation of Energy Separation in the Vortical Wake Flow of a Turbine Nozzle Cascade

This paper describes the observation, measurement, and computation of vortex shedding behind a cascade of turbine nozzle guide vanes that have a blunt trailing edge. At subsonic discharge speeds, periodic wake vortex shedding was observed at all times at a shedding frequency in the range 7–11 kHz. At high subsonic speeds the wake was susceptible to strong energy redistribution. The effect was greatest around an exit Mach number of 0.95 and results are presented for that condition. An unusually cold flow on the wake centerline and hot spots at the edges of the wake were measured. These were found to be a manifestation of Eckert–Weise effect energy separation in the shed vortex street. Experimental identification of these phenomena was achieved using a new stagnation temperature probe of bandwidth approaching 100 kHz. Using phase-averaging techniques, it was possible to plot contours of time-resolved entropy increase at the downstream traverse plane. Computational work has been undertaken that gives qualitative confirmation of the experimental results and provides a more detailed explanation of the fine scale structure of the vortex wake. The topology of the wake vortical structures behind blunt trailing-edged turbine blades is becoming clearer. These measurements are the first instantaneous observations of the energy separation process occurring in turbine blade wake flows. This was also the first demonstration of the use of the probe in the frequency, Mach number, and temperature ranges typical of operation behind the rotors of high-performance turbomachines such as transonic fans.

Investigation of three-dimensional flowfield at the exit of a turbine nozzle

The objective of the research reported in this article is to gain a detailed understanding of the flowfield at the exit of turbine nozzles. The flowfield was measured at two axial locations downstream of the nozzle of a single-stage turbine with a miniature five-hole probe. An area traverse was carried out to resolve the flowfield accurately, including the nozzle wake, secondary flow region, horseshoe vortex, and losses. All three components of the velocity, stagnation pressure, static pressure, and pitch and yaw angles have been resolved accurately. A distinct vortex core has been observed near the tip and at the hub. The indications are that the horseshoe vortex and the passage vortex have merged to produce a single-loss core region. Roughly, a third of the blade height passage near the tip and a third of the blade height near the hub is dominated by secondary flow phenomena. Only the middle third of the nozzle behaves according to design. The nozzle wake decay is compared to the wake decay of other blades. The nozzle wake decays much faster than a compressor cascade wake, an annular turbine nozzle cascade wake, or a turbine nozzle wake with a large rotor-stator spacing. This faster decay is attributed to effect of the downstream rotor at very small rotor-stator spacing (20% of nozzle axial chord). These and other data are presented, interpreted, and synthesized to understand the nozzle flowfield.

Effect of Boundary Layer Fences Attached to Casing of Annular Turbine Nozzle Cascade.

This study aims at reducing secondary flow and associated losses in total pressure in axial-flow turbines. Boundary layer fences were attached to the casing wall of an annular nozzle cascade. Hot-wire and pneumatic measurements were carried out downstream of the cascade for 7 different protruding heights of the fences. The optimum height of the fences was approximately 1/4 of the inlet boundary layer thickness. The optimum fences reduced the passage-mass-averaged loss by 14 percent from the unfenced blading value. This reduction resulted from local loss reductions within and near the blade wake, which compensated for the installation of the fences. The maximum underturning, induced by the secondary vortices, was markedly attenuated with the use of optimum fences.

Heavy Fuels Ash Deposit Formation and Removal in Water-Cooled High-Temperature Gas Turbines

Experimental data obtained in heavy fuels operation of a gas turbine simulator with a water-cooled, transonic turbine nozzle cascade are presented. The ash fouling is characterized by the rate of decrease of the aerodynamic throat area. Particular attention is given to the cleanability of the ash deposits. A simple heat transfer analysis was performed to assist in evaluating the data. The rate of ash fouling in the water-cooled nozzle was found to be of the same order of magntude as for conventional air-cooled designs. Cleanability, both on and off-line, was found to be significantly enhanced, thus making the water-cooled gas turbine an attractive alternative for heavy fuels applications.

Heat Transfer Analysis In A 2D Turbine Cascade

A Navier Stokes solver for compressible turbulent flow is applied to the prediction of heat transfer distributions in a two-dimensional highly loaded transonic turbine nozzle cascade. The solver is an implicit scalar factored ADI time marching scheme. Centred finite difference are adopted for spatial discretization, and artificial dissipation terms are added on both explicit and implicit side. Non reflecting boundary conditions are chosen for the far field boundaries; the computational domain is discretized with a structured 'C mesh. Two different algebraic turbulence models are implemented: a standard Baldwin Lomax and an RNG based one. Numerical results are compared with experimental data. Nomenclature

Closure to “Discussions of ‘An Investigation of the End-Wall Boundary Layer of a Turbine-Nozzle Cascade’” (1957, Trans. ASME, 79, pp. 1805–1806)

Closure to “Discussions of ‘An Investigation of the End-Wall Boundary Layer of a Turbine-Nozzle Cascade’” (1957, Trans. ASME, 79, pp. 1805–1806)